The present invention relates to a proportional valve assembly, particularly for supplying compressed air on a glassware molding machine.
In the glass industry, hollow glass articles are produced on so-called I.S. molding machines, which comprise a number of air-powered operating units, at least some of which are supplied and controlled by respective proportional valve assemblies.
In most applications, the proportional valves comprise an elongated outer jacket in which the inlet, outlet, and exhaust ports are formed; and a sleeve housed in the jacket and comprising a number of projections mating in fluidtight manner with the inner surface of the jacket.
To open/close the various ports, the sleeve is normally moved axially inside the jacket by an electric linear actuator, on the one hand, and, on the other, by a reaction defined by the combined action of a spiral return spring and the pressurized fluid supplied by the valve assembly. The spring normally comprises an end portion inserted inside a seat in the jacket; and an opposite end portion fitted to an axial projection integral with the sleeve.
Though widely used on conventional molding machines, known valve assemblies of the above type fail to meet the requirements of modern glassware molding machines in terms of response speed and repeat positioning. This is due to the reaction exerted on the sleeve not always being purely axial, and in many casesxe2x80x94due to machining, positioning and assembly tolerances and/or deformation under loadxe2x80x94comprising a radial component varying uncontrollably in direction and intensity. The presence of components crosswise to the traveling direction of the sleeve affects the slide resistance of the sleeve inside the jacket, and produces localized wear which impairs response time, sealing, and operating precision, and which obviously increases alongside an increase in fluid supply pressure.
It is an object of the present invention to provide a proportional valve assembly designed to provide a straightforward, low-cost solution to the above drawbacks.
According to the present invention, there is provided a proportional valve assembly, particularly for supplying compressed air on a glassware molding machine, the assembly comprising a tubular body having an axis and inlet and outlet ports for an operating fluid; at least one movable slide member housed in said tubular body; a linear actuator for exerting control action on said slide member; and reaction means for exerting on the slide member an opposing action in opposition to the control action, and for moving said slide member along said axis; characterized by also comprising relatively moving means interposed between said reaction means and said slide member.